How to edit textures with texture paint in Cycles like GLSL in Blender internal?
You could try the viewport shading to Material, it will display image textures as mixed in the shader node setup:



From the wiki:


  Material: a simplified version of the entire material is drawn using
  GLSL shaders. This uses solid lighting, and also is mostly useful for
  editing, painting and mapping textures, but while seeing how they
  integrate with the material.